Tic Tac Toe - Invent Your Own Computer Games with Python - page 171

10 - Tic Tac Toe
'1 2 3 4 5 6 7 8 9'.split() expression on the left side of or would return
False as expected, and then we would evaluate the expression on the right side of the or
operator. But when we pass 'X' (which would be the value in move ) to the int()
function, int('X') would give us an error. It gives us this error because the int()
function can only take strings of number characters, like '9' or '42' , not strings like 'X'
As an example of this kind of error, try entering this into the shell:
>>> int('42')
42
>>> int('X')
Traceback (most recent call last):
File "<pyshell#3>", line 1, in <module>
int('X')
ValueError: invalid literal for int() with base
10: 'X'
But when you play our Tic Tac Toe game and try entering 'X' in for your move, this
error doesn't happen. The reason is because the while loop's condition is being short-
circuited.
What short-circuiting means is that because the expression on the left side of the or
keyword (move not in '1 2 3 4 5 6 7 8 9'.split() ) evaluates to True , the
Python interpreter knows that the entire expression will evaluate to True . It doesn't matter
if the expression on the right side of the or keyword evaluates to True or False , because
only one value on the side of the or operator needs to be True .
Think about it: The expression True or False evaluates to True and the expression
True or True also evaluates to True . If the value on the left side is True , it doesn't
matter what the value is on the right side. So Python stops checking the rest of the
expression and doesn't even bother evaluating the not isSpaceFree(board, int
(move)) part. This means the int() and the isSpaceFree() functions are never
called.
This works out well for us, because if the expression on the right side is True then
move is not a string in number form. That would cause int() to give us an error. The
only times move not in '1 2 3 4 5 6 7 8 9'.split() evaluates to False
are when move is not a single-digit string. In that case, the call to int() would not give
us an error.
An Example of Short-Circuit Evaluation
Here's a short program that gives a good example of short-circuiting. Open a new file in
the IDLE editor and type in this program, save it as truefalsefizz.py , then press F5 to run it.